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I decided to take apart my MBA by myself to clean it up. I know that it is wrong — it was my decision and I knew what to do. I did everything very carefully. After I put everything back my MBA works fine, even better than before, BUT the machine's serial number is missing and sound has been disabled (there is no startup sound, no sound in headphones, and the built-in speakers do not show at all in system info).

I didn't replace a motherboard, just took the MBA apart and put it back together again. So I can't see how I could have damaged something on the motherboard that would explain the serial number disappearing or sound not working.

Resetting a new serial number is not a huge deal — just need to use Blank Board Serializer, but I can't understand why it is missing.

Could the two be related? What could have happened that explains this.

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I'd start with resetting the NVRAM and SMC (in that order) as there's really no harm in trying that first. Before following the steps below, ensure you have no external hardware connected and that you're using the built-in keyboard.

Reset the NVRAM

Reset the NVRAM on your MBA as follows:

  1. Shut down your machine. Yes, a full shut down, not just logging out.
  2. Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
  3. Hold those keys down until your MBA reboots again (i.e. you hear the startup chime a second time).
  4. Let go of the keys and let your MBA reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Reset the SMC

To reset the SMC on your MBA, follow these steps:

  1. Shut down your MBA
  2. Keep the power cable plugged in.
  3. Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
  4. Let go
  5. Turn your computer back on with the power button.

After resetting both the NVRAM and SMC check to see if either (or both) issues still persist.

If they do, then run some hardware diagnostics next.

Testing hardware

The diagnostics you run depend on your model of MacBook:

Once doing all this you should be able to identify/narrow down the cause.

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